Crude oil is typically distilled to produce a variety of components that can be used directly as fuels or that are used as feedstocks for further processing or upgrading. In what is known as atmospheric distillation, a heavy residuum is produced typically that has an initial boiling point of about 650° F. (343° C.). This residuum is typically referred to as atmospheric residuum or as an atmospheric residuum fraction.
Atmospheric residuum fractions tend to collect a relatively high quantity of various metals, sulfur components and nitrogen components relative to the lighter distillation fractions as a result of the distillation process. Because these metal, sulfur and nitrogen components are relatively undesirable in various fuels, they are typically removed by various catalytic hydroprocessing techniques.
In some instances, the atmospheric residuum is further distilled under vacuum, i.e., at a pressure below atmospheric pressure, to recover additional distillation fractions. At vacuum conditions, additional lighter fractions can be recovered without adding to various problems encountered in atmospheric distillation such as coking of the heavy fraction components. The heavy residuum recovered in vacuum distillation of the atmospheric residuum is typically referred to as vacuum residuum or a vacuum residuum fraction, and typically has an initial boiling point of about 1050° F. (566° C.). This vacuum residuum is generally higher in metals, sulfur components and nitrogen components than atmospheric residuum, and as was the case with atmospheric residuum, removal of these components is typically carried out by catalytic hydroprocessing.
Catalytic hydroprocessing of atmospheric and vacuum residua is carried out in the presence of hydrogen, using a hydroprocessing catalyst. In some processes, hydroprocessing of residua is carried out by adding a diluent or solvent.
U.S. Pat. No. 3,617,525 discloses a process for removing sulfur from a hydrocarbon fraction having a boiling point above about 650° F. (343° C.). In carrying out the process, the hydrocarbon fraction is separated into a gas oil fraction having a boiling point between about 650° F. (343° C.) and about 1050° F. (566° C.), and a heavy residuum fraction boiling above about 1050° F. (566° C.). The gas oil fraction is catalytically hydrodesulfurized until the gas oil fraction contains less than 1 percent sulfur. The hydrodesulfurized gas oil is then used to dilute the heavy residuum fraction, and the diluted heavy residuum fraction is catalytically hydrodesulfurized, producing fuels or fuel blending components reduced in sulfur content. The process is considered to provide an increased catalyst life and to use a smaller reactor volume compared to typical processes.
U.S. Pat. No. 4,302,323 discloses a process for upgrading a residual petroleum fraction in which the residual fraction is mixed with a light cycle oil and hydrogen and the mixture sent through a catalytic hydrotreating zone containing a hydrotreating catalyst and then a hydrocracking zone containing a hydrocracking catalyst. Upgraded products are then separated from the effluent of the hydrocracking zone. The light cycle oil boils in the range of from 400° F. (204° C.) to 700° F. (371° C.), has a high aromatic content, and is high in nitrogen. It is considered that the light cycle oil acts more as a diluent rather than as a hydrogen donor and that the addition of the light cycle oil resulted in a substantial increase in the yield of premium products such as distillate fuels.
U.S. Pat. No. 4,421,633 discloses a combination hydrodesulfurization and hydrocracking process. The feedstock can be atmospheric residuum or vacuum residuum, which is mixed with a solvent that is a recycled distillate boiling at about 400° F.-700° F. (204° C.-371° C.), considered to be equivalent to a FCC light cycle oil. The process uses a mixture of large pore and small pore catalysts such as large and small pore sulfided Ni—W catalysts. The process converts the higher boiling point residua to lower boiling point hydrocarbons by forming distillate and naphtha while removing heteroatoms, metals and carbon residuals from the higher boiling point residua.
There is a need to further develop processes for hydroprocessing heavy hydrocarbon oils to produce fuel grade products. It is also particularly desirable to provide hydroprocessing processes with improved selectivity to desired products. For example, it is desirable to provide hydroprocessing processes that crack molecules boiling at or above 1050° F. (566° C.) (also referred to as a “1050° F.+(566° C.+) fraction” herein) into molecules boiling below 1050° F. (566° C.) (also referred to as a “1050° F.− (566° C.−) fraction” herein), while minimizing the formation of “C4−” hydrocarbon compounds (i.e., hydrocarbon compounds having four carbons or less), and coke byproducts.